1. Field of the Invention
The present invention generally relates to a system for controlling the speed of the fuel pump.
2. Description of Related Art
Automotive fuel pump systems have been widely used though out the automotive industry. Typically most fuel pumps run at the highest pressure and maximum flow rate at all times to reduce the amount of fuel vapor for vehicle hot restart and provide sufficient fuel in a wide open throttle condition. However, running at the highest fuel pressure and flow is not efficient and negatively affects the life of the fuel pump.
In fuel pump applications, it is desirable to vary the amount of fuel provided from the fuel pump depending on the engine performance requirements. For instance, a vehicle at full throttle may require 90 liters of fuel per hour, while at idle the vehicle may consume only 3 liters of fuel per hour. There are a number of problems associated with the return of fuel from the high pressure, high temperature engine area to the relatively low pressure and low temperature fuel tank area. In an idle condition, the high pressure and temperature of the fuel being returned to the fuel tank causes substantial amounts of fuel vapor to be generated. The vapor must be vented from the fuel tank area which may, additionally, raise environmental issues.
One solution is controlling the amount of fuel delivered to supply only the amount of fuel used. The amount of fuel delivered is dependent on the fuel pressure generated by the fuel pump. Generally, the fuel pressure is related to the speed of the pump.
One method used to vary pump speed to control fuel pressure uses a voltage drop resistor. The resistor is selectively connected to the fuel pump voltage supply to control the voltage provided to the pump motor thereby changing the pump speed. Although this method reduces fuel pump wear, little energy is saved as the additional voltage is dissipated across the voltage drop resistor. Further, the additional heat energy created by the voltage drop resistor must be dissipated.
Another method used to vary pump speed thereby affecting fuel pressure includes modulating the driving signal. A pulse width modulator can be used to vary the duty cycle of the pump driving voltage thereby changing the pump speed. Although this method also reduces fuel pump wear and some energy is saved, the power and frequency of pulses required to drive the pump cause radio frequency interference problems for other vehicle components. Further, the use of a pulse width modulator in the control circuit increases system complexity and cost.
In view of the above, it is apparent that there exists a need for an improved system for controlling the speed of the fuel pump.